For an air conditioning system (hereinafter referred to as AC system), a gas compressor is used. A gas compressor operates upon receiving external supply of power. Therefore, a gas compressor becomes a load with respect to an external power source. In the case where an AC system is mounted on a vehicle, for example, a power source is often an engine of the vehicle. In the engine, when a gas compressor is operating, it is necessary to optimally perform control, for example, fuel injection with an engine control device (hereinafter referred to as engine control unit (ECU)) in accordance with the load of the gas compressor. Thus, estimation of a torque upon operation of the gas compressor is performed (for example, see JP 3921430 B).